A variety of retinal diseases cause vision loss by destruction of the outer retinal vasculature and certain outer and inner retinal layers of the eye. The inner retina is also known as the neuroretina. The outer retinal vasculature is comprised of the choroid and choriocapillaris, and the outer retinal layers are comprised of Bruch's membrane and retinal pigment epithelium. The outer portion of the inner retinal layer that is affected is the photoreceptor layer. Variable sparing of other inner retinal layers, however, may occur. These spared inner retinal layers include the layers of the outer nuclei, outer plexiform, inner nuclei, inner plexiform, amacrine cells, ganglion cells, and the nerve fibers. The sparing of these inner retinal layers allows electrical stimulation of one or more of these structures to produce sensations of formed images.
Prior efforts to produce vision by electrically stimulating various portions of the retina have been reported. One such attempt involved a disk-like device with retinal stimulating electrodes on one side and photosensors on the other side. The photosensor current was to be amplified by electronics (powered by an external source) within the disk to power the stimulating electrodes. The device was designed to electrically stimulate the retina's nerve fiber layer via contact upon this layer from the vitreous cavity. The success of this device is unlikely because it must duplicate the complex frequency modulated neural signals of a nerve fiber layer which runs in a general radial course with overlapping fibers from different portions of the retina. Accordingly, the device would not only have to duplicate a complex and yet to be deciphered neural signal, but would also have to be able to select appropriate nerve fibers to stimulate that are arranged in a non-retinotopically correct position relative of the incident light image.
Another attempt at using an implant to correct vision loss involves a device consisting of a supporting base onto which a photosensitive material, such as selenium, is coated. This device was designed to be inserted through an external sclera incision made at the posterior pole and would rest between the sclera and choroid, or between the choroid and retina. Light would cause an electric potential to develop on the photosensitive surface producing ions that would then theoretically migrate into the retina causing stimulation. However, because this device has no discrete surface structure to restrict the directional flow of the charges, lateral migration and diffusion of charges would occur thereby preventing an acceptable image resolution capability. Placement of the device between the sclera and choroid would also result in blockage of discrete ion migration to the photoreceptor and inner retinal layers. This is due to the presence of the choroid, choriocapillaris, Bruch's membrane and the retinal pigment epithelium layer, all of which would block passage of these ions. Placement of the device between the choroid and retina would still interpose Bruch's membrane and the retinal pigment epithelium layer in the pathway of discrete ion migration. As the device would be inserted into or through the highly vascular choroid of the posterior pole, subchoroidal, intraretinal and intraorbital hemorrhage would likely result along with disruption of blood flow to the posterior pole.
Another retinal stimulating device, a photovoltaic artificial retina device, is disclosed in U.S. Pat. No. 5,024,223. This patent discloses a device inserted into the potential space within the retina itself. This space, called the subretinal space is located between the outer and inner layers of the retina. The disclosed artificial retina device is comprised of a plurality of so-called surface electrode microphotodiodes (“SEMCPs”) deposited on a single silicon crystal substrate. SEMCPs transduce light into small electric currents that stimulate overlying and surrounding inner retinal cells. Due to the solid substrate nature of the SEMCPs, blockage of nutrients from the choroid to the inner retina can occur. Even with fenestrations of various geometries, permeation of oxygen and biological substances is not optimal.
U.S. Pat. No. 5,397,350 discloses another photovoltaic artificial retina device. This device is comprised of a plurality of so-called independent surface electrode microphotodiodes (ISEMCPs) disposed within a liquid vehicle, for placement into the subretinal space of the eye. The open spaces between adjacent ISEMCPs allow nutrients and oxygen to flow from the outer retina into the inner retina. ISEMCPs incorporate a capacitive layer to produce an opposite direction electrical potential to allow biphasic current stimulation. Such current is beneficial to prevent electrolysis damage to the retina due to prolonged monophasic stimulation. However, like the SEMCP device, the ISEMCP depends upon light from the visual environment to power it, and so the ability of this device to function in low light environments is limited. The ISEMCP also does not provide a way to address localized variations in the sensitivity to electrical stimulation of surviving retinal tissue. Accordingly, there is a need for retinal implants that can operate effectively in low light environments and are capable of compensating for variations of retinal sensitivity within an eye.